1. Field of the Invention
This invention relates generally to line start permanent magnet motors and, more specifically to line start motors having magnets outside starting cages and magnetization of such motors.
2. Description of the Related Art
Conventional line start permanent magnet motors are designed with the magnets inside the induction starting cage. In these motors, large block magnets can be used. The magnets inside the cage are only partially protected from demagnetization during starting (when the rotor flux can be 180 degrees out of phase with the stator flux at very high current levels). When the line voltage at full frequency is applied to a stopped motor winding, the air gap magnetic field revolves very rapidly past the stationary rotor magnets and thus (although strong pulsating torques exist) develops no net torque. The motor is instead started by currents flowing in the starting cage exactly as in an induction motor. Resulting induction torques, however, are not always high enough to start some motors because the induction cage is magnetically interrupted and often electrically interrupted as well. Additionally, the stator flux must traverse the thickness of the magnets which is usually considerably larger than the air gap and behaves similarly. The result is poor performance as an induction motor. After this type of conventional motor is started and synchronized, little or no current flows in the induction cage.
In these permanent magnet motors, the cage torque must overcome the "drag" loading of the magnets in addition to the load. This drag has two components, both of which vary with speed and air gap flux density. The first drag component is from the stator core losses that are induced by the rotating flux of the magnets which varies with the square of speed and air gap flux density. The second drag component is the torque which results from the voltages induced in the stator windings by the rotating flux of the magnets which also vary with the square of the air gap flux density and as a function of speed. Because the frequency of the resultant voltage in the stator windings does not equal the line frequency, the power system appears to the stator as a short circuit. Hence current flows in the stator, and power is dissipated in the stator windings as a result.
Moreover, the rotor cage can never be fully magnetically uniform and complete because that would "short circuit" the magnetic flux of the magnets allowing little or no magnetic flux to reach the air gap for interaction with the stator currents. One method of avoiding a magnetic short circuit is to extend the slots in the rotor down towards the magnets to create a "flux barrier." The result is that the cage does not produce torque as effectively as it would if complete. The flux barrier to induction flux represented by the magnet is another major factor in degrading cage performance. Thus, fabricating a flux barrier permanent magnet motor with inside magnets results in an ineffective induction motor.